# Product of group subsets

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In mathematics, one can define a product of group subsets in a natural way. If S and T are subsets of a group G then their product is the subset of G defined by

Note that S and T need not be subgroups. The associativity of this product follows from that of the group product. The product of group subsets therefore defines a natural monoid structure on the power set of G.

If S and T are subgroups of G their product need not be a subgroup. It will be a subgroup if and only if ST = TS and the two subgroups are said to permute. In this case ST is the group generated by S and T, i.e. ST = TS = <ST>. If either S or T is normal then this condition is satisfied and ST is a subgroup. Suppose S is normal. Then according to the second isomorphism theorem ST is normal in T and ST/ST/(ST).

If G is a finite group and S and T and subgroups of G then the order of ST is given by the product formula:

Note that this applies even if neither S nor T is normal.

In particular, if S and T (subgroups now) intersect only in the identity, then every element of ST has a unique expression as a product st with s in S and t in T. If S and T also permute, then ST is a group, and is called a Zappa-Szep product. Even further, if S or T is normal in ST, then ST is called a semidirect product. Finally, if both S and T are normal in ST, then ST is called a direct product.